After you kill the first guy, go to the left and kill the second nurse. Take the kevlar and the MP5 with all the ammo. Close the gate behind you and step back from it for like 5 steps(this is important otherwise they will not come from the front and will get stuck in the left room). All clones will come to you from the front and no one will shoot you if stay behind the left part of the gate. Shoot close to the gate so you don't hit the bars. When the clone is dead, take his gun. Repeat the process for every clone and no one will ever shoot you. Maybe it's a known cheese but I just figured it out today.